a) Field of the Invention
The invention is directed to a movable or traveling large-area welding machine for welding plastic sheets spread out upon a substratum and which overlap over the width of a zone, having a frame formed by a first leg extending in the direction of travel of the machine and a second leg arranged transversely to the latter. The first leg has means for welding the two sheets in the aforementioned zone as well as contact-pressure or press-on means running on this zone for loading the welded sheets against the substratum, and the second leg is provided with traveling rollers arranged on a common axle. The aforementioned press-on means include a press-on roller which is connected via a common drive with the traveling rollers which are arranged so as to be offset to the latter in the direction of travel as well as transversely thereto.
b) Background Art
Such welding machines are used to weld together plastic sheet webs having a large surface area and a considerable length, particularly for sealing roofs, floors and walls. For this purpose, such a machine has, for example, a hot air nozzle which acts between the overlapping edges of the sheets. The exiting hot air melts the two edges which are then pressed together and welded with one another.
A machine of the type indicated above is already shown and described in EP-A-O 279 306. Although this machine has proven substantially successful, certain difficulties are revealed with respect to advancing the device on the spread out sheets and in keeping to the traveling track along the overlapping zone, chiefly when traveling over uneven surfaces of the underlying base or substratum.
The problems of constructing such a machine will be discussed briefly in this connection. As already mentioned, this machine must run along long sheet webs in a straight line as far as possible. For this purpose, it would not be advisable to provide a steering mechanism in the manner of an automotive vehicle, i.e., a steering axle, because, apart from the required expenditure on construction and control means, play would be introduced in the steering members which would constitute a severe impediment to straight running. However, if no steering mechanism were provided, the rollers of the machine, all of which are driven according to the cited prior art, would have to have exactly the same outer diameter. In this respect, it must be considered that at least the traveling rollers have a friction or traction lining at their circumference, that is, an applied ring of material which can be resiliently compressed so that straight running is made considerably more difficult. It is likewise impossible for the traveling rollers at the front as seen in the traveling direction to run along loosely instead of being driven, since the rear press-on roller (which is then driven alone) is laterally offset relative to the track of the front traveling rollers. Therefore, the rolling resistance of the latter would lead to a turning moment in the traveling plane, resulting in a relatively rapid divergence from straight running. Another requirement for such a traveling welding machine consists in that upsetting such as bulging or the occurrence of waves or "bubbles" in the spread out sheet webs due to the longitudinal movement must be prevented, if possible. However, such upsetting can be caused even by slight differences in the circumferential speed of the press-on means and the traveling rollers.